PRIM/LAND IMPACT

Prims, Prim Equivalent, Land Impact... a too-long guide

by Jenni Darkwatch

from the Second Life forums

Because there's a ton of misinformation and
even more woe-to-me, here's a bit of a guide - in laymans terms. For
everyone who's interested in the details please do feel free to look
over at the mesh forum.Disclaimer:
This information represents my own opinion on how things work, and is
largely the result of experimentation and reading various posts from
people way more talented than myself. Especially Drongle McMahon, Gaia
Clary and Chosen Few.

WHY IT IS IMPORTANT TO KNOW ABOUT LIThere's
a good question: Why would anyone besides creators even care? Because
with mesh and with the new accounting system that comes with it, the
following scenario is quite possible. I built a simple house, it's 11
prims as you can see in the build floater:A
few things to note: The house was built using regular box prims and a
cylinder. It's not exactly well-built either. After all there's no door
and no windows More
important to note and not immediately obvious is that the build floater
doesn't list "Prims" but "Land Impact". For old objects that
distinction is irrelevant. But creators are now increasingly using mesh
items and/or items which contaim partial mesh content. Mesh accounting,
i.e. Land Impact, is vastly different from the good old prim based
accounting.Here is the same house, but using Land Impact (mesh-type) accounting:It's
the exact same building, but it only has a land impact of 6. In old
terms, it means that even though I used 11 prims to build this house,
the simulator only subtracts 6 from my parcel allowance.For
consumers this can quickly become difficult to understand, because they
might buy this house and the ad might say "11 prims, 6 Land Impact" or
more common on the marketplace "11 prims, 6 Prim Equivalent".

PRIM ACCOUNTING - PAST AND PRESENTWe
all know the good old region prim count. It's used everywhere. If your
region has a maximum capability of 1000 prims, then 1000 prims is what
you can rez. Regardless of whether the prim is a regular prim or a
sculpted prim, and regardless of their size.

With
mesh, there's now a new accounting system: Land Impact (LI). It
calculates three values for any prim: Download Weight, Physics Weight
and Server Weight. It also calculates a fourth value, Display Weight,
but that one is not used for Land Impact (LI) calculations.Unfortunately,
old viewers can not properly display Land Impact, and that's a problem.
Because if a customer would buy that house from the first paragraph,
old viewers would show both houses as using 11 prims, even though the
second one only subtracts 6 from your parcel allowance. Old viewers
cannot distinguish the two houses.

HOW LAND IMPACT WORKSIt
gets a bit technical here by necessity, but bear with me please. LI is
calculated for each and every prim in an object, regardless of whether
it's a simple prim, a sculpt or mesh. By default, only mesh is forced to use the new LI system. This is where one misconception stems from: That mesh is less efficient.In
the newer viewers, Land Impact for objects can be examined in the build
floater. Here's what it looks like for the good old 0.5m cube:Sorry
for the low contrast. The advanced floater appears when clicking on the
"More info" link in the build floater. LL changed the terminology
across the board. No longer does land have a prim allowance, it has a LI
allowance. The distinction is significant.In
the advanced floater there is a section called "WEIGHTS OF SELECTED",
listing four values: Download, Physics, Server and Display. These terms
are not explained very well in any LL reference that I could find, so
here's what they mean based on Linden comments and resident research:Download: It literally is a measure of how much any prim impacts the network side. More complex objects have higher download weights. The
download weight corresponds to the size of most (but not all) prim
types. The bigger a prim, the higher the download weight. Box prims stay
the same at all sizes. Torii don't. As Drongle has kindly pointed out,
the download weight is kind of in-between server and viewer, as it
relates to bandwidth use.Physics:
A server side measurement. This relates to the complexity of the
physical appearance of a prim, which is not the same as the visual
appearance. More on that below in the physics types section.Server:
This seems to be largely script weight, again server side. Unscripted
prims seem to have a server weight of 0.5 regardless of shape or type.Display:
This is the only value that is not relevant for LI. It's meant as a
measure of render engine impact and as such is clientside. Because this
value is highly subjective it's not used for LI impact calculations.The LI value of any object is always the highest of the three values Download, Physics or Server, and it always gets rounded up.You
will notice that the simple box has a highest value of 0.5. That gets
rounded up to an LI of 1. What happens if we link two boxes together?As
we all know, it counts as two prims as seen in the build floater. But
the highest weight of the two boxes is 1.0. That is because by default, non-mesh prims use the old accounting system of just adding the number of prims to a prim count total.That's where it gets interesting. It is possible to opt-in to the new LI accounting system, like I have done here:Et
voila, the land impact dropped to one. In other words, these two boxes
only count as one single prim against a regions old-style prim
allowance. And that's why LL has removed reference to prim count in most
places including land/parcel info floaters. It's now all called LI. How
to opt in to the new system? Change the physics shape of ANY object or
prim in a linkset to anything other than prim. If even one prim
in a linkset is opted in to the new accounting, the whole linkset
automatically gets opted in. Also, if a linkset contains even just one
mesh, the whole linkset opts-in to the new LI accounting.

WHY WE HAVE TWO ACCOUNTING SYSTEMSThere
always has been a problem in SL. A prim isn't a prim. For servers, a
torus is a horrible thing. It's physical shape is complicated, and it's
relatively complex to describe the shape of a torus. Boxes are simple.Most
simple objects would benefit from the new LI accounting system. But
some would not. Let's opt-in a pair of tori to the new accounting
system:I
admit up-front I cheated. I only opted the root prim into the new
accounting by changing its physics shape to Convex Hull. More on these
physics shapes later. As you see, this can be a real "Oh damn!" moment.
These two prims have a LI of 37! Or in old terms, they count the same as
an object made out of 37 prims.That's
why we currently have two accounting systems. The old "a prim is a
prim" system and the new LI system. If LL had forced the new system in, a
lot of people would have had a lot of their things returned to
inventory. When experimenting with the new accounting system it's a good
idea to either be in a sandbox or on a parcel with plenty available
LI/prims. In a sense, the new LI accounting exposed just how inefficient
prim-style building can be in some cases.Keep in mind, old viewers will not see LI. They only see prims.

PHYSICS SHAPES EXPLAINEDPhysics
shapes are what make SL useable for avatars. Without them we could not
walk into houses, over bridges and so on. Physics are not free, they
have to be calculated. The servers handle that. Many objects in SL
really would not need to have any physical shape, or could at least use a
very simple shape. Next to script memory use, physics are one area
where we as residents have direct influence on sim performance now.We
currently have three distinct physics shapes: Prim, Convex Hull and
None. Meshes can have their own physics shapes, I'll ignore them for
simplicity and just use a torus for explanation.Physics shape type: PRIMI've
upscaled the torus enough so I can stand inside. On the right I enabled
showing the physics shape of the torus (in Develop->Render
Metadata->Physics Shapes). You see that the physical shape matches
the visual appearance of the torus quite nicely.The example torus above is using the default physics shape type for old-style prims, aptly named Prim. Lets switch to Convex Hull and see what that does. To illustrate, I turned the torus so the opening is on top.Physics shape type: CONVEX HULLYou
see that my avatar stands on the hollow part, it does not fall in.
Again enabling display of the physics shape shows why. When switching to
Convex Hull, the physical shape ignores the hole and closes it. Convex
Hull can be thought of as a gift-wrap shape. It just follows the outline
and ignores any holes. Here's a side-by-side of the two physics shapes:You
can clearly see the difference when looking at the triangles. Also,
Convex Hull does not conform nearly as precise to the actual prim shape.
It's a lot easier on the physics simulation though.Physics shape type: NONEThe last new shape doesn't require an image, as there would be nothing to see. Physics shape type "None"
removes any physics shape from a prim. It effectively turns the prim
phantom. This can be used in linksets to have individual prims phantom,
removing the need to either script objects for that or have a separate
linkset for all phantom prims.Linksets
can have a mix of all physics shape types. I'd expect that the better
creators use that new ability to optimize builds for low server impact.
But I'm not holding my breath for every creator doing it. Most don't
give a damn anyway, it's easier to blame LL for lag.I'd
like to warn again: Be careful when experimenting with this - it's
quite possible to blow LI through the roof especially with complex
builds. Go experiment in a sandbox if you're low on free prims.

WHAT DOES IT MEAN FOR SLThe
new accounting system gives us yet again a new tool in our arsenal to
make SL a pleasant place and cut down on lag. I'm sure many here are
familiar with the statistics floater (in Advanced->Performance
Tools->Statistics Bar). It gives, among other things, a snapshot of
the sims health:Specifically the time display is of interest, as it tells us where the sim is using its processing time.The three weights that make up LI are more or less directly tied to three of the time values in this display:Download Weight affects (vaguely) Net Time.Physics Weight directly affects Physics Time.Server Weight (at this point in time) mostly affects Script Time.This
is overly simplified of course, but it illustrates just why we, as
residents, should care. Not every performance problem can be attributed
to poorly created content of course, that would be unfair to creators
who put a lot of time and effort in to making quality products. A lot of
content is badly built regardless, partly because we didn't have the
tools to effectively measure the impact of our creations on SL.Now
we, creators and residents, have better tools at hand. As residents it
allows us to make more educated choices when buying things in SL.

PROBLEMS WITH THE NEW SYSTEM

The new LI accounting system has a number of flaws, as near as I can tell.An object thats set Phantom still gets the full physics penalty, even though at that point it should not affect the physics engine at all.Since
the new system is opt-in unless you use mesh, it does somewhat
encourage "cheating" by setting all objects that benefit from it to the
new LI system and leaving all objects that do not benefit out of the LI
system. Personally I don't see that as much of a problem, but it definitely is a problem.The
new system also places A LOT of weight on scripting. Scripts kill any
objects LI quite fast. No idea what the reasoning behind that is.Last
but not least: The new system is not explained very well anywhere.
There's no central information hub on it. The documentation that does
exist is often outdated, incomplete or both.Especially
for creators the new system is problematic. People with older viewers
cannot see it. People with newer viewers may not know about it. The
marketplace doesn't support any information on it.

Anyway...
if you read this far, my apologies for the lengthy pamphlet. I hope
it'll help understand what's going on. ---

About Me

I was a teacher long before I began operating my La Galleria business full time, and you can never take the teacher out of a teacher. I like to help my customers learn skills that will make their Second Lives easier, more creative, and fun -- I have taken some of my help notecards and Picks and put them in my instructional web pages on modifying and troubleshooting. Take a peek!